818 
$ 3. In the second place we investigated the behaviour of: four 
preparations, these being dextro-, respectively laevogyratory muca- 
piles consisting of muscovite-lamellae crossing at 45° or 60°. The 
composing lamellae were cut from a muscovite-crystal in such a 
way, that their longer side was parallel to the optical axial plane 
of the mineral, their shorter edge thus being parallel to its. plane of 
erystallographical symmetry. The central part of the complexes-com- 
posed of hexagonally arranged lamellae, manifested in convergent 
polarized light between crossed nichols the almost perfect axial 
image of a uniaxial crystal endowed with circular polarisation ; in 
the interference-image of the dextro- or laevogyratory complexes 
built up by lamellae crossing under 45°, there appeared only a 
single dark beam interrupted in the central part of the image,:while 
also the coloured rings showed a somewhat elliptical distortion with 
a slight spiral constriction in the immediate vicinity of the dark 
beam. For the rest, the optical properties of the preparation appeared 
to vary quite continuously in all azimuths, being almost the same 
in all directions. The Ronrexnpatterns obtained are reproduced. in 
fig. 2 and 3 of Plate I, in the right position with respect to that 
Fig. 2a. Stereographical Projection of the normal Diffraction-pattern of a dextro 
or laevogyratory Complex of Muscovite-lamellae, crossing at 60°. 
